Implement file attachments feature

I've added support for a single file to be attached to any message.

Note that it's a little hacky in some places. I'm open to suggestions about how to improve it.

The data per message is now double because of this. I'm wondering if it's a good idea to restrict the size of the total data instead of the message and attachment individually. That might make more sense if a multiple file attachment feature is added.

Also, if ZeroBin is to support files, it might be a good idea to check the file size in JavaScript before uploading.

Tested in Firefox 18.0.1 on Ubuntu 12.10. I don't currently have compatibility checks.

Let me know what you think.